Make metadata structured and pass its type as a generic parameter of Storage

[ikopylov]

Metadata can be abstracted as this was done with Key. Possible trait can look like this:

trait Meta: Eq + Clone + Debug + Serialize + Deserialize {
   // Ordering type that specifies version of the value (the large value, the later is version)
   // Example: timestamp
   type VersioningKey: Ord + Eq + Clone + Debug;
   // Returns version of record from its metadata
   fn get_version(&self) -> VersioningKey;
   
   type MetaFilter: Debug;
   // Returns true when this metadata has equal fields with `filter`
   // This function is needed for `read_with`
   fn matches_with(&self, filter: &MetaFilter) -> bool;
}

This trait can allow to customize values versioning (it can be timestamp, creation time or any other versioning source). It will allow to remove explicit timestamp argument from write and delete functions. It will allow to build RangeFilter on top of VersioningKey.